1. Field of the Invention
The present invention relates to a piezoelectric device for converting an input mechanical quantity into an output electrical quantity, and vice versa.
2. Description of the Related Art
As generally known in the art, piezoelectric devices are typically used to convert an input mechanical stress into an output electric charge (voltage) and also to convert an input electric voltage into an output mechanical distortion. In the latter case, the device is sometimes referred to as an electrostrictive device. The term xe2x80x9cpiezoelectric devicexe2x80x9d as used herein is to be interpreted in its broadest sense, as encompassing an electrostrictive device as well. Similarly, the term xe2x80x9cpiezoelectric materialxe2x80x9d as used herein is to be interpreted in its broadest sense as encompassing an electrostrictive material as well. When the output of the piezoelectric device is a mechanical distortion, such distortion can be used to generate mechanical force, displacement or vibration.
Conventionally, piezoelectric devices have been used for various purposes. For example, in the field of optics or precision engineering, there is a progressive demand for actuators capable of adjusting lengths and/or positions of the optical path on a sub-micron order, or for sensors capable of detecting a fine mechanical displacement. There have been various proposals directed to such demands in conjunction with application of piezoelectric devices. Specifically, there is known unimorph- or bimorph-type piezoelectric device which undergoes bending or flexural deformation upon application of an input electrical voltage, and which can be used for ink jet printing heads, acoustic radiators (i.e., loudspeakers and the like), vibrators, etc. The unimorph- or bimorph-type piezoelectric device can also be used to convert a bending or flexural deformation into an electrical voltage, e.g., a microphone or sensor. With reference, for example, to ink jet printing heads including a unimorph- or bimorph-type piezoelectric device, in view of customers"" or end users"" requirement for high-quality and high-speed printing performance, it is highly desirable to realize a high density piezoelectric device having a minimized size, which operates at a low driving voltage and yet provides a satisfactory response characteristic and an operational reliability.
For realizing the above-mentioned desired properties of the piezoelectric device, because the substrate in the region of each piezoelectric transducer functions as a diaphragm, it has been generally considered necessary for the entire substrate to have a reduced thickness. In many instances, however, the reduced thickness of the substrate as a whole cannot be achieved without lowering the mechanical strength of the substrate. Thus, in order to simultaneously satisfy the seemingly incompatible requirements for the reduced thickness and a satisfactory mechanical strength of the substrate, U.S. Pat. No. 5,210,455 assigned to the assignee of this application discloses an improved piezoelectric device which includes a locally thin-walled substrate and piezoelectric transducers formed on, and integrated with the thin-walled regions of the substrate.
As particularly disclosed in U.S. Pat. No. 5,210,455, the locally thin-walled substrate may be formed by laminating a relatively thick green sheet of a first ceramic layer having window-like openings therein, and a relatively thin green sheet of a second ceramic layer without such openings. After integrating the two ceramic layers by firing to form the substrate, the openings in the first ceramic layer cooperate with the second ceramic layer to define cavities having thin-walled bottom walls which are constituted by the second ceramic layer. Furthermore, the piezoelectric transducers may each include a first electrode layer, a piezoelectric layer and a second electrode layer which are sequentially laminated with each other. The piezoelectric transducers are arranged on those outer surface regions of the substrate opposite to the relevant cavities, and are integrated with the substrate by firing.
The piezoelectric device according to the above-mentioned proposal proved to be highly advantageous in that the locally thin-walled regions of the substrate opposite to the respective piezoelectric transducers make it possible to achieve a satisfactory operation of the transducers, and further in that the remaining, relatively thick region of the substrate serves to realize a sufficient mechanical strength of the substrate as a whole.
Nevertheless, it would be highly desirable to provide a further improved piezoelectric device which is superior in the operational characteristics, and which yet retains the above-mentioned advantages. Having thus conducted thorough experiments and investigations based on the piezoelectric device disclosed in U.S. Pat. No. 5,210,455, the inventors reached a recognition that a significant improvement can be achieved by carefully taking into consideration the physical and/or chemical properties, structure and behavior of the substrate and the piezoelectric transducers at or near the thin walled regions of the substrate.
First of all, in the piezoelectric device according to the inventors"" earlier proposal, the substrate and the piezoelectric transducer are generally composed of materials which are different in chemical composition and hence in the coefficient of thermal expansion. Thus, during manufacture of the piezoelectric device, in particular during cooling which follows the sintering for forming the piezoelectric transducer on the substrate or the firing for integrating the substrate and the piezoelectric transducer, undesirable residual stresses tend to be caused in the piezoelectric transducer, which may deteriorate the operational characteristics of the device as a whole. According to a recognition reached by the inventors, a highly improved piezoelectric device can be realized by effectively compensating for the difference in shrinkage of the materials for the substrate and the piezoelectric transducer.
Furthermore, in the piezoelectric device according to the inventors"" earlier proposal wherein a common substrate is provided with an array comprising a plurality of piezoelectric transducers, when neighbouring transducers are simultaneously actuated, it is often difficult to achieve a sufficient amount of displacement of the thin-walled regions of the substrate in a direction perpendicular to the general surface of the substrate, and a satisfactory volumetric change of the cavity. Besides, the operation of the transducers may be affected by the operation of adjacent transducers, making it difficult to uniformly achieve a desired displacement amount of the thin-walled regions of the substrate. According to another recognition reached by the inventors, a highly improved piezoelectric device can be realized by eliminating or suppressing undesired interaction of the neighbouring transducers.
Specifically, it has been confirmed that when the piezoelectric device according to the inventors"" earlier proposal is used, for example, as an actuator for an ink jet printing head, there may be instances wherein the ink filled within the cavity cannot be discharged with a sufficient amount or at a high speed, or wherein it is difficult to achieve a satisfactory printing quality. According to still another recognition reached by the inventors, significantly improved ink discharge characteristics can be achieved by realizing a sufficient displacement characteristic of the piezoelectric transducer and a satisfactory volumetric change of the cavity. It has also been recognized that a significantly improved printing quality can be realized by eliminating or minimizing fluctuation in the displacement characteristics of the piezoelectric transducers and hence in the discharge amount and speed of ink.
The present invention thus contemplates providing a highly improved piezoelectric device on the basis of the above-mentioned recognitions.
It is a primary object of the present invention to provide a highly improved piezoelectric device which makes it possible to achieve a sufficient displacement amount of the thin-walled region of the substrate and a satisfactory volumetric change of the cavity.
It is another object of the present invention to provide a highly improved piezoelectric device which is capable of effectively relieving the stresses in the piezoelectric transducer, which may arise from the difference in shrinkage of the substrate and the piezoelectric transducer due to the difference in the coefficient of thermal expansion of the materials, or from the shrinkage of the piezoelectric transducer upon sintering of the transducer on the substrate.
It is still another object of the present invention to provide a highly improved piezoelectric device provided with an array comprising a plurality of piezoelectric transducers, which is capable of eliminating or suppressing undesired interaction of the neighbouring transducers.
Briefly stated, the piezoelectric device according to the present invention comprises a ceramic substrate with an outer surface and an inner surface, of which the inner surface is locally formed with at least one cavity which is defined by at least one peripheral wall and a bottom wall having a reduced thickness. The device further comprises at least one film-type piezoelectric transducer which is formed integrally with the substrate and arranged on the outer surface opposite to the bottom wall. The outer surface of the substrate has a thick-walled region adjacent to and surrounding the peripheral wall of the cavity. The thick-walled region is formed with at least one groove which extends along the peripheral wall of the cavity. The groove has a depth which is greater than the thickness of the bottom wall. The groove is arranged so as to permit simultaneous displacements of the bottom wall and the thick-walled region between the cavity and the groove, to thereby cause a volumetric change of the cavity.
The above-mentioned arrangement of the piezoelectric device according to the present invention features the formation of at least one groove in the relatively thick-walled region of the substrate adjacent to and surrounding the peripheral wall of the cavity. Because the groove extends along the peripheral wall of the cavity and has a depth which is greater than the thickness of the bottom wall of the cavity, it is possible to provide a higher compliance of the relatively thick-walled substrate region adjacent to the peripheral wall of the cavity, with which the thin-walled regions of the substrate can be readily deformed upon firing shrinkage during manufacture of the device. Therefore, despite the difference in the coefficient of thermal expansion of the materials for the substrate and the piezoelectric transducer, the transducer during manufacture of the device can be subjected to the firing shrinkage in a relatively unrestricted sense, thereby minimizing the residual stresses in the transducer and assuring highly improved operational characteristics of the device.
Moreover, when the piezoelectric device according to the present invention includes an array of piezoelectric transducers formed on a common substrate, the groove in the relatively thick-walled region of the substrate can be advantageously situated between neighbouring transducers to provide a low vibration transmissibility of the substrate between adjacent transducers. This serves to effectively eliminate or suppress undesirable interaction or crosstalk between the transducers, and makes it readily possible to uniformly achieve a desired amount of displacement of the thin-walled regions of the substrate in a direction perpendicular to the general surface of the substrate.